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Autotrophic bacteria are a wide range of self-sustaining organisms which are capable of producing their own food. These microorganisms grow by using inorganic nutrients and are important in cycling of inorganic compounds. Autotrophs not only satisfy their own needs for reduced carbon monomers from inorganic matter but can also feed the already existing heterotrophs. Thus, autotrophic organisms are also called primary producers.
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Key Terms: Autotrophs, Photosynthesis, Photoautotrophs, Chemoautotrophs, Bacteria, Energy, Food, Environment
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Types: Photoautotrophs
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A wide range of microorganisms, as well as the green plants, algae and protists, are phototrophic. They use light as an energy source in the process of photosynthesis. The consequence of this mechanism is the generation of a proton motive force that can be used in the synthesis of ATP and reducing power (e.g., NADPH).
Most photoautotrophs use the energy stored in ATP and the electrons in NADPH to assimilate carbon dioxide as a carbon source for biosynthesis. These phototrophs are called photoautotrophs. There are also phototrophs able to use organic compounds as carbon sources with light as an energy source; they are called photoheterotrophs.
The types and examples of different types of photoautotrophic bacteria are:
Aerobic Photosynthetic Bacteria
This type of bacteria is an example of photosynthetic bacteria which uses oxygen to carry out photosynthesis. Cyanobacteria or blue-green algae is an example of photosynthetic bacteria. In this type, water molecules are used as electron donors and oxygen is produced in the reaction. They have photosynthetic pigments like Chlorophyll but they lack chloroplasts.
Reaction: 6CO2 + 12H2O + light → C6H12O6 + 6CO2 + H2O
These bacteria are filamentous and colonial which are capable of nitrogen fixation. The autotrophic bacteria examples of this category are Nostoc, Anabaena, etc.
Anaerobic Photosynthetic Bacteria
Anaerobic photosynthetic bacteria is the type of photosynthetic bacteria which uses hydrogen sulphide or thiosulphate instead of water as a reducing agent. These bacteria use bacteriochlorophyll during the process of photosynthesis. Examples of anaerobic photosynthetic bacteria include green sulphur bacteria, purple non-sulphur bacteria, purple sulphur bacteria, etc.
Reaction: CO2 + 2H2S + light → [CH2O] + 2S + H2O
Purple Sulphur Bacteria
Purple Sulphur Bacteria are found in the hot sulphur springs and stagnant water. These bacteria are anaerobic or Microaerophillic bacteria that make use of hydrogen sulphide or thiosulphates during the process of photosynthesis.
Purple Sulphur Bacteria contain bacteriochlorophyll ‘a’ and ‘b’ which are found in the plasma membrane. These bacteria can reduce harmful waste from the environment and lower the amount of hydrogen sulfide, methane and toxic compound present in water inlets. Examples include Chromatiaceae, Nitrosococcus, etc.
Purple Non-sulphur Bacteria
The Purple Non-Sulfur Photosynthetic Bacteria constitute a non-taxonomic group of adaptable organisms like photoheterotrophs and photoautotrophs or chemoheterotrophs which can switch from one mode to another depending on conditions. These bacteria use hydrogen(H2) as a reducing agent instead of hydrogen sulphide(H2S). They are very important since they produce polyphosphates, vitamins, pigments, hydrogen, extracellular nucleic acids and growth-promoting substances.
Green Sulphur Bacteria
Thriving in anoxic environments, these gram-negative rods or spherical shaped bacterias are found in deep oceans, thermal vents and in extremely low light conditions. Green Sulphur Bacteria have sulphide, hydrogen or ferrous ion as electron donors. They also have bacteriochlorophyll ‘c’, ‘d’ and ‘e’ along with bacteriochlorophyll ‘a’. They fix carbon by reverse tricarboxylic acid and are similar to the PSI system of higher plants.
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Types: Chemoautotrophs
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Discovery of autotrophy in chemolithotrophic bacteria was of major significance in the advance of our understanding of cell physiology because it showed that CO2 could be converted to organic carbon without photosynthesis.
Chemoautotrophs are important in the cycling of inorganic compounds on Earth, including methanogens, which produce methane, and nitrifiers, which convert ammonia to nitrate. Depending on the source from which they draw energy, they are classified into various types like sulfur bacteria, hydrogen bacteria, ferrous bacteria, nitrogen bacteria, methanotrophs, etc. They play an important role in the recycling of nutrients such as nitrogen, phosphorus, sulfur, iron, etc.
Sulphur bacteria: This type of bacteria oxidizes hydrogen sulphide or thiosulphates to molecular sulphur or sulphates. E.g. Beggiatoa, Sulfolobus, etc.
Reaction: 2H2S + O2 → 2H2O + 2S + Energy
Nitrogen bacteria: Nitrogen bacteria convert ammonia to nitrite and then to nitrate and energy is released. E.g. Nitrosomonas, Nitrobacter
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Reaction: NH3 + O2 → NO2 + H2O + Energy (Nitrosomonas)
2NO2 + O2 → NO3 + Energy (Nitrobacter)
Hydrogen bacteria: As the name implies, they oxidise molecular hydrogen, where aerobic hydrogen-oxidizing bacteria use O2 as an electron acceptor, whereas anaerobic hydrogen bacteria use nitrogen dioxide or sulphate as an electron acceptor. E.g. Helicobacter pylori, Hydrogenobacter thermophilus, Hydrogenovibrio Marinus, etc.
Reaction: 2H2 + O2 → 2H2O + Energy
Methanotrophs: They use methane as a carbon source to derive energy and can be aerobic or anaerobic. Aerobic methanotrophs oxidize methane to formaldehyde, which is then utilized in various pathways to form organic compounds whereas, anaerobic methanotrophs utilize other compounds as electron acceptors. E.g. Methylomonas, Methylococcus capsulatus, etc.
Iron bacteria: Iron Bacteria oxidise ferrous ions to ferric ions. They are present in iron-rich environments like hot lava beds, and hydrothermal vents. E.g. Thiobacillus ferrooxidans, Geobacter metallireducens, Zetaproteobacteria, Gallionella, Ferrobacillus, etc.
4FeCO3 + O2 + 6H2O → 4Fe(OH)3 + 4CO2 + Energy
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Things to Remember
- Autotrophic bacteria are capable of producing their own food.
- Autotrophic bacteria are of two types, Photoautotrophic Bacteria and Chemoautotrophic Bacteria.
- Photoautotrophic Bacteria produce food and energy by photosynthesis.
- Chemoautotrophic Bacteria produce their food and energy by using chemical compounds.
- Autotrophic bacteria are mostly free-living organisms that are found in soil, water bodies like oceans etc.
- These type of bacteria can sustain their life in any extreme environment and can remain inside the body of any multicellular organism.
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Sample Questions
Ques. What is the importance of autotrophs in ecology? (3 Marks)
Ans. Autotrophs are present in all ecosystems. They take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates. Thus, they meet their requirements easily and can be constitutive around the world.
Autotrophic organisms are primary producers and, as a consequence, they are at the pyramidal base of the ecosystems. Thus, heterotrophs depend on autotrophs for the energy and raw materials they need. On the other hand, autotrophs, as a consequence of their poor requirements, have greater adaptability; thus, they are especially important in oligotrophic environments, like oligotrophic lakes, glaciers and ice, acid waters, geothermal systems, etc.
Ques. What are autotrophic bacteria? (2 Marks)
Ans. Autotrophic bacteria are those bacteria that can synthesize their own food. They perform several reactions involving light energy (photons) and chemicals in order to derive energy for their biological sustainability.
Ques. How do photoautotrophic bacteria make their own food? (2 Marks)
Ans. They make their own food like plants. Photoautotrophic bacteria may perform oxygenic photosynthesis or anoxygenic photosynthesis. Photosynthetic bacteria are used as biofertilizers, for bioremediation, wastewater treatment and purification of polluted water.
Ques. What does Purple Sulphur Bacteria consist of? (3 Marks)
Ans. Purple Sulphur Bacteria contain bacteriochlorophyll ‘a’ and ‘b’ which are found in the plasma membrane. These bacteria can reduce harmful waste from the environment and lower the amount of hydrogen sulfide, methane and toxic compound present in water inlets. Examples include Chromatiaceae, Nitrosococcus, etc.
Ques. How do the autotrophs take energy? (2 Marks)
Ans. Autotrophs take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates. They meet their requirements easily and become constitutive around the world
Ques. Discuss autotrophic bacteria and heterotrophic bacteria. (3 Marks)
Ans. Heterotrophic bacteria- obtain energy from organic molecules.
Autotrophic bacteria- make their own food by converting light energy, chemical energy or inorganic substances into food.
1) Chemoautotrophs- are autotrophic bacteria which obtain their nutrition from inorganic compounds.
2) Photoautotrophs- are autotrophic bacteria that absorb light energy to carry out photosynthesis to create cellular energy.
Ques. Explain Hydrogen bacteria (3 Marks)
Ans. Hydrogen bacteria oxidize molecular hydrogen and use O2 as an electron acceptor, whereas anaerobic hydrogen bacteria use nitrogen dioxide or sulphate as an electron acceptor. E.g. Helicobacter pylori, Hydrogenobacter thermophilus, Hydrogenovibrio marinus, etc.
2H2 + O2 → 2H2O + Energy
Ques. How are Bacteria differentiated on the basis of colour? (3 Marks)
Ans. Different bacteria have different colours due to the presence of different types of light-capturing pigments. Chlorophyll is one such type of pigment that imparts green colour to the bacteria and plant leaves.
The green sulphur bacteria found in deep oceans are an example of such a type of bacteria. They perform photosynthesis similar to that of higher plants. There are also purple sulphur bacteria and purple non-sulphur bacteria which do not contain chlorophyll but other pigments. The reason for the prefix sulphate before Bacteria is because they use hydrogen sulphide or thiosulphate during photosynthesis.
Ques. Explain Methanotrophs. (2 Marks)
Ans. Methanotrophs use methane as a carbon source and to derive energy. They are either aerobic or anaerobic. Aerobic methanotrophs oxidize methane to formaldehyde, which is then utilized in various pathways to form organic compounds.
Ques. What is photosynthesis? (2 Marks)
Ans. The green plants use carbon dioxide and water present in the atmosphere and soil to produce glucose with the help of light energy provided by the sun. The glucose produced consists of molecules formed of 6 carbon atoms 12 hydrogen atoms and 6 oxygen atoms. This process for the production of glucose is known as photosynthesis.
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